I got a fuel check valve (with AN threads) and 2 barb adapters. The fittings have matching cone shapes. I don't know if that will seal when the adapters are screwed on tight. The fuel pressure is about 350 kPa. Would you put on something like Loxeal thread sealer, or a fuel-resistant o-ring?
Just tighten them up, adding anything as a sealer may (if it breaks down) induce a leak, it is only 50 psi. Joints like that are used in refrigeration all the time at far higher pressures and there it is usual to put a smear of oil on the surfaces before tightening. Many fuel filters also have cone fittings and run at similar pressures without anything added.
I fitted it as is, and it hasn't leaked. I since learned that AN fittings work like this, but if you got ORB couplers instead, they need o-rings.
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